Thats a terrible way to open a 9v battery. Why not bend the metal tabs on the bottom out and push the casing off? That way there is no risk of cutting into a battery and spraying chemicals all over the place.

If you were to fold the metal tabs over on the battery, you could substitute them for a AA in a pinch, they wouldn't last as long but they will still work for more information, look up Kipkays instructable on a 9V battery hack. :) happy hacking